1. Field of the Invention
This invention relates generally to pulling cable and, more specifically, to pulling lightweight cable such as optical fiber cable.
2. Description of the Prior Art
The cable placement techniques for copper and aluminmum telephone power cables are a well developed art. Typically, the first step is pushing fish wire through a conduit to connect to a steel pulling cable. The pulling cable usually passes through a pulley system to produce a mechanical advantage. The pulling cable firmly connects to one end of a conductor cable which is to be pulled through the conduit. The conductor cable is fed into the conduit while an operator on the opposite end uses the pulley system to pull the cable through the conduit. With suitable lubricant and carefully constructed conduit, one can pull a 31/2 inch diameter telephone cable up to 1,800 feet.
With the development of lightweight fiber optical cable, the use of the conventional pulling method is both cumbersome and expensive. Fiber optical cables are advantageous to use over copper conduit cables because fiber optical cables have a much smaller diameter and much lower density than copper conductors even though both transmit the same amount of information. A typical size for fiber optical cables is 1/4 inch diameter and an overall density of less than 1 gram/milliliter. This size fiber optical cable will transmit as much information as 2 to 4 inch diameter copper cable with a density of 6 to 10 grams/milliliter.
To minimize the number of repeater stations and splices with fiber optical cable, it is preferred that fiber optical cable be installed in very long runs, usually in the range of 5 kilometers to 10 kilometers. Pulling cable up to 10 kilometers is a problem because it is well known that the pulling force to pull the cable through the conduit is equal to the sum of the frictional forces on both the pulling cable and the cable to be installed. The total frictional forces are generally a linear function of the length of the conduit through which the cable is pulled. Generally, the total frictional force resisting the pulling of the cable through a conduit becomes excessive as the length of the conduit increases beyond 2 or 3 kilometers. Even in the straight smooth conduits with use of lubrication, attempts to pull lengths beyond 2 or 3 kilometers have been unsuccessful.
A second problem resulting from the use of smaller diameter fiber optical cable is that the cables oftentimes become wedged between larger cables in the conduit. Cable manufacturers have attempted to strengthen the optical cables by making optical cable of high tensile strength to withstand the tension forces produced in long pulls. To make a high tensile strength cable requires incorporation of special strengthening fibers into the body of the cable. These fibers increase the strength but also increase the size and cost of the cable. Typically, the tensile strength can be increased to about 400 lbs. for a 1/4 inch diameter cable.
The present system and apparatus solves the prior art problem of pulling lightweight cable by impelling a small diameter plug along a tube to pull the fiber optical cable therein by use of fluid pressure.